1. Technical Field
This is in the field of radio frequency receivers for astronomical observations.
2. Related Technology
Radio astronomy began in 1932 with the discovery of radio emission from the Galactic Center at the relatively long wavelength of 15 m (20 MHz) by Karl Jansky, as described in Jansky, K. G. 1932, Proc. Inst. Radio Engrs., Vol. 20, 1920. This pioneering work was followed by the innovative research of Grote Reber at frequencies ranging from 10-160 MHz (30-2 meter wavelength) in the 1940s that closely tied radio astronomy to the broader field of astronomy and astrophysics, as described in Reber, G. 1940, ApJ, 91, 621; Reber, G. 1944, ApJ, 100, 279; Reber, G. & Greenstein, J. L. 1947, The Observatory, 67, 15; Reber, G. 1949, S&T, 8, 139; and Reber, G. 1950, Leaflet of the Astronomical Society of the Pacific, 6, 67.
However, the requirement for impractically large single radio antennas or dishes to obtain resolution at long wavelengths (resolution θ˜λ/D, where θ is the angular resolution in radians, λ is the observing wavelength in meters, and D is the diameter of the observing instrument in meters), quickly pushed the new field of radio astronomy to higher frequencies (shorter wavelengths). Thus, since increasing the antenna diameters was severely limited by cost and mechanical considerations, the field moved toward achieving higher resolution by decreasing the observing wavelength.
As early as 1946, Ryle and Vonberg and Pawsey and collaborators began to use interferometric techniques that relied on large arrays of simple dipoles or widely separated individual, small dishes to increase the effective diameter D without greatly increasing the cost. Even then, distortions introduced into the incoming radio signals by the Earth's ionosphere made imaging at long wavelengths difficult and appeared to place a rather short upper size limit to D at frequencies less than 100 MHz (wavelengths greater than 3 m) of about 5 km. Thus, the move to higher frequencies, even for interferometry, continued until, by the 1970 s, relatively few long wavelength radio astronomy telescopes were still operating at frequencies below 100 MHz. Some exceptions include the Ukrainian UTR-2, the 38 MHz survey with the Cambridge Low-Frequency Synthesis Telescope, and the Gauribidanur Radio Observatory (GEETEE).
The Tee Pee Tee (TPT) Clark Lake array was built by William C. (Bill) Erickson on a dry lake in the Anza-Borrego desert east of San Diego, Calif. (Erickson, Mahoney, & Erb 1982). The TPT was also limited to a maximum baseline D of 3 km because of concerns about ionospheric distortion.
An array of antennas that would measure interstellar radiation at long wavelengths with high resolution was first proposed by R. A. Perley of the National Radio Astronomy Observatory and W. C. Erickson of the University of Maryland in 1984., in “A Proposal for a Large, Low Frequency Array Located at the VLA Site”, 14 Apr. 1984.
Perley and Erickson envisioned studying large scale emission around individual galaxies and clusters of galaxies, studying the low brightness regions of radio galaxies and quasars, radio sky surveys, studies of source variability at low frequencies to distinguish between intrinsic and instellar variation, studying the spectra of extragalactic objects, and studying normal spiral galaxies. They further envisioned studying pulsars, the galactic center, HII regions, flare stars, star clusters, galactic background emission, interstellar propagation effects, and exotic objects. Such a long wavelength high resolution array would also be useful for solar system observations, including the sun, the planets, the moon, and solar wind turbulence. They proposed operation in the 75 MHz range, with tenability over 20 MHz to allow operation in interference free bands.